The present invention relates to a high toughness heat-resistant steel, a turbine rotor and a method of producing the same, and more particularly, to improvements in material of the high toughness heat-resistant steel used for high/low pressure combined type turbine rotor and the like which are especially suitable for a power plant aiming at a large volume and high efficiency.
In general, in a steam turbine in which a plurality of turbine rotors are mechanically coupled together, materials for the rotors are selected in accordance with steam conditions used from the high pressure side to the low pressure side. For example, CrMoV steel (ASTM-A470 (class 8)) or 12Cr steel (Japanese Patent Application Publication No.60-54385) is used as a material for turbine rotor used at the side of high temperature (550 to 600.degree. C.) and high pressure, and NiCrMoV steel (ASTM-A471 (classes 2 to 7)) including 2.5% or more of Ni is used as a material for turbine rotor used at the side of low temperature (400.degree. C. or lower) and high pressure.
In a recent power plant achieving large volume and high efficiency, a so-called high/low pressure combined type turbine rotor in which a high pressure side portion and a low pressure side portion are integrally formed of the same material has attracted attention, in view of miniaturization of the steam turbine and simplification of the structure.
However, since the conventional steel for the above-described turbine rotor is not a material intended to be used under the condition which covers all of the requirements from the high pressure side to the low pressure side, if such a conventional steel is used to form the high/low pressure combined type turbine rotor, the following problems are present:
1): In the case of CrMoV steel, although it is excellent in creep rupture strength in a high temperature region of about 550.degree. C., its tensile strength and toughness are not always satisfactory in a low temperature region, and ductile fracture, brittle fracture or the like are likely to occur. Thus, to counteract this, it is necessary to reduce stress acting on the lower pressure portion of the turbine rotor. As a result, the size of a blade mounted at a low pressure stage, especially at the final stage is restricted. From this point of view, it is difficult to increase the volume of a power plant. Further, also with respect to high temperature creep rupture strength, CrMoV steel does not always satisfy the condition of high temperature (about 600.degree. C.) and high pressure of steam at the entrance of a turbine that is required for enhancing the efficiency of a power plant.
2) In the case of 12Cr steel, this steel is superior to CrMoV steel in high temperature creep rupture strength, and thus can satisfy the above-described condition for the steam at the entrance of the turbine. However, since this steel does not have enough toughness, a countermeasure also is required as in the case of CrMoV steel, and the size of blade that can be mounted at the low pressure stage is limited.
3) In the case of NiCrMoV steel, although this steel has excellent tensile strength and toughness at the low temperature region, its creep rupture strength is not always satisfactory, and since the strength of this steel used at the high pressure side is not sufficient, it is necessary to limit the degree of high temperature of the steam at entrance of turbine, and it is difficult to enhance the efficiency of the power plant.
As described above, when a high/low pressure combined type turbine rotor is formed using the conventional steel, there is a problem that a great restriction can not be avoided when an effort is made for increasing volume and enhancing the efficiency in a steam turbine in which a long low pressure final stage blade is mounted.